An electronic key system (a smart key system) by electronic control using an ID (IDentity) or security authentication has been put to practical use in order to improve convenience and operability with respect to locking and unlocking of the vehicle doors, engine start, and the like. The smart key system is a system that performs authentication of a mobile device carried by a user by an in-vehicle device that is incorporated in a vehicle via wireless communication, so as to control functions, such as locking and unlocking of the vehicle and engine start. The user can lock or unlock the vehicle and start the engine simply by carrying the mobile device.
The smart key system controls locking and unlocking of the vehicle and engine start by wireless communication between the mobile device and the in-vehicle device. Therefore, the mobile device needs to receive a signal, which is sent at any time from the in-vehicle device, without fail. To receive a signal at any time, it suffices that a receiver is on standby at all times. However, the power consumption increases if it is set on standby at all times. Because the mobile device is battery-operated, suppression of power consumption is effective for extending the battery life.
In a conventional smart key, to suppress power consumption of the mobile device, there is a system that has a power saving state where power consumption is lower than a communicable state, and shifts to the communicable state at a predetermined timing to realize the communication. For example, Patent Literature 1 describes a technique in which, during standby, the mobile device shifts to a power saving state for a time determined according to the user's operation of the mobile device, and, during authentication, the mobile device shifts to a power saving state for a predetermined time or a time notified by the in-vehicle device. Furthermore, Patent Literature 2 describes a method of suppressing power consumption by extending a transmission interval of signals from the in-vehicle device after authentication of the mobile device, thereby reducing the number of times transmission and reception are performed between the in-vehicle device and the mobile device.